Monitoring of Residual Disease in Chronic Myelogenous Leukemia by Quantitative Polymerase Chain Reaction and Clinical Decision Making

We thank Goldman et al and Lion for their letters to the editor in response to our commentary.1-1 We agree with both that polymerase chain reaction (PCR) technology has undoubtedly added valuable information to our current knowledge and understanding of minimal residual disease (MRD) in chronic myelogenous leukemia (CML) and other hematologic malignancies. We are also pleased that both letters address 2 basic issues regarding the clinical use of PCR: the operational (ie, clinical utility of such testing) and the biological (ie, the correlation of PCR results with the biology of MRD in CML). However, we believe that their enthusiasm for the clinical use of PCR is not supported by the available data.

Regarding the operational issue, even Goldman et al admit that PCR assays do not routinely achieve high sensitivity in practice and that their sensitivity can vary between specimens. Furthermore, qualitative PCR assays, especially at single time points, are not useful for predicting relapse. Meanwhile, quantitative PCR assays, such as those used for the serial evaluation of MRD in CML, have yet to meet the criteria expected of any laboratory test applied in clinical medicine: (1) standardization of the test across different laboratories; (2) reproducibility of positive and negative predictive values; and (3) reliable sensitivity and specificity. Because quantitative PCR testing is cumbersome, technically demanding, and highly dependent on internal standards and on the personnel conducting these tests,1-2such assays should especially not be used to guide clinical treatment until they are standardized and proven to be acceptably reproducible, specific, and sensitive.

The biological nature of MRD in hematologic malignancies is intriguing. Residual disease is a dynamic process under the control of as yet unidentified mechanisms and characterized by significant variability among individuals. Furthermore, fluctuations in the level of measurable MRD do exist and, despite the technical pitfalls associated with PCR, such fluctuations have to be followed serially and quantitatively to allow any meaningful interpretation.1-3 However, does the lack of detection of BCR-ABL transcripts in a patient with MRD in CML mean that the patient is cured, or conversely, does aBCR-ABL–positive PCR assay herald imminent relapse? The answer is most likely no to both questions.1-1,1-3,1-4 Kurzrock et al,1-5 for example, found that BCR-ABL transcripts were not detected by reverse transcriptase-PCR (RT-PCR) in 10 of 18 patients treated with interferon-α and in complete cytogenetic remission. Does PCR negativity correlate with improved survival? The answer is unclear. Talpaz et al1-4 analyzed patients with CML in complete cytogenetic remission who were being treated with interferon-α and were also BCR-ABL negative by RT-PCR. Using clonogenic assays combined with RT-PCR, Talpaz et al1-4 could still identify individual colonies that expressedBCR-ABL fusion transcripts. More recently, at the American Society of Hematology meeting in December 1998, Talpaz presented an update of these 7 patients with CML in whom long-term interferon-α treatment was discontinued and who were in unmaintained complete cytogenetic remission at a median follow-up duration of 10 years (range, 7 to 12 years). Four of the patients had detectable residual disease (30, 387, 73, and 20 BCR-ABL transcripts/μg protein, respectively). Other investigators reported similar observations.1-6,1-7 Thus, patients can apparently remain in long-term remission or probably even be cured and still express high levels of BCR-ABL transcripts (severalfold higher than those reported by Biernaux et al1-8 in healthy individuals).

Can a threshold of residual disease detected by PCR be defined above which relapse appears likely? Cross et al1-9 analyzed 91 patients by qualitative PCR. Twenty-eight patients with a positive PCR result were further studied by competitive PCR for quantitation of theBCR-ABL transcript. Among the 17 patients without cytogenetic relapse, BCR-ABL transcript levels ranged from 10 to 800 μg/mL. Among the 11 patients who relapsed, transcript levels ranged from 1,600 to 700,000 μg/mL, although these levels were determined at the time of cytogenetic relapse. In another study, Hochhaus et al1-10 quantified residual BCR-ABL transcripts by RT-PCR in 31 patients with CML who had achieved a complete cytogenetic remission after interferon-α therapy. BCR-ABL transcripts were detected in all patients with BCR-ABL transcript levels ranging from less than 10 to greater than 14,000/μg RNA. Moreover,BCR-ABL/ABL ratios for all patients ranged from 0.00075% to 3.6%, with a median of 0.024%. Among the 6 patients who later had a relapse (25 patients with residual disease did not have a relapse),BCR-ABL/ABL ratios were greater than 0.024%. Whether a similar threshold can be confirmed in other studies remains to be determined.

Is there any evidence that early intervention based on PCR monitoring is beneficial? Studies by van Rhee et al1-11 and Raanani et al1-12 suggest so, because both reported that molecular monitoring identified early relapse and that therapeutic intervention before the onset of hematologic relapse was associated with an increased probability of a response to donor lymphocyte infusions. However, both studies can be criticized, because they were not randomized, had small patient numbers, and did not clearly define early relapse. In fact, patients they considered to be in early relapse were, for the most part, patients in cytogenetic relapse, and only 2 in each study were in molecular relapse. Therefore, the studies by van Rhee et al1-11 and Raanani et al1-12 provide no evidence that initiation of therapy at molecular relapse is any better than initiation of therapy at cytogenetic relapse. We are still left with a lack of data as to the usefulness of therapy that is based on positive PCR results.

In conclusion, we again agree that PCR studies aimed at detecting, quantifying, and characterizing MRD have contributed valuable information to our understanding of MRD in hematologic malignancies. The presence of BCR-ABL transcripts in patients who are probably cured is undoubtedly interesting. If these transcripts represent residual disease, then the identification of the mechanism responsible for its suppression is very important. However, we again assert that, thus far, it has not been established that detection of MRD in CML by PCR can be used clinically. Until PCR testing has been standardized and proven reliable and until randomized studies have proven that treating molecular relapse is indeed beneficial for patients with CML, PCR-based results should be interpreted only within the confines of well-defined clinical trials. Consequently, any generalized guidelines for using PCR tests to guide treatment appear premature and should be viewed critically.